The invention relates to apparatus for bringing into harmony two flows of fluid and the amounts of fluid which result from such flows over a predetermined period of time, or for achieving a given difference between the two flows or quantities of fluid, under sterile conditions, that is to say, avoiding germ infection or other contamination of the fluids. Such an apparatus can be used with advantage inter alia in certain medicinal treatment processes, if the fluid which is drawn from one patient is to be continuously replaced by another fluid to the same degree or with a defined difference between the two amounts of fluid.
An example of a process of this kind is exchange blood transfusion in which blood is taken from the patient and replaced by blood from an outside source. In order to avoid inadmissible fluctuations in the total volume of blood in the circulation of the patient, during this process, the amounts of blood drawn from and supplied to the patient must be as closely parallel to each other as possible.
A second example is infusion therapy in conjunction with forced diuresis, which is used for example in certain cases of poisoning in order to remove the poison from the body as quickly as possible by increased urine excretion which is triggered off by medicament means. In this case, it is necessary for the high loss of fluid to be replaced as regularly as possible, for which purpose the intravenous infusion of suitable amounts of fluid is frequently employed. The infused fluid must be matched in regard to its quantity to the quantity of urine which is excreted.
A third example is haemofiltration (also called haemodiafiltration) in which a filtrate is sucked out of the blood of the patient through a filter, the filtrate substantially comprising water and low-molecular blood constituents, including also urine-related substances which would normally have to be excreted by way of the kidneys. This process is used in particular for replacing a failing kidney function. The major part of the fluid drawn off must be replaced by intravenous infusion of a substitution solution, in parallel with the withdrawal of the filtrate.
The above-listed examples always involve the problem of introducing a substitute fluid (blood or infusion solution) into the patient's circulation, in accordance with the withdrawal or automatic excretion of another fluid (blood, urine, filtrate). The supply of substitute fluid must obviously be effected under absolutely sterile conditions. The latter requirement severely limits the technical means which can be employed, and means that the sterile substitute fluid must be supplied to the patient from a sterile container through a sterile tube and possibly further sterile connecting and communicating members. For reasons of safety and operating simplification, it is nowadays generally usual for the members which are employed for such purposes to be in the form of single-use articles which are supplied in a sterile condition by the manufacturer and are thrown away when they have been used once.
Balancing, that is to say, achieving a condition of harmony between the amounts of fluid which are supplied and withdrawn, possibly taking into account an intentional difference between the two amounts of fluid, is effected in many cases by observation and manual control of the process, for example by the fluid which is withdrawn being collected in a calibrated vessel and the supply of substitute fluid being regulated in accordance with the values which are read off on the calibrated vessel. This process which requires continuous observation is very labour-intensive. Disadvantageous consequences can result for the patient, due to errors in reading off the values or due to other human error.
The present invention is therefore based on the problem of providing a process and apparatus with the aim of automating balancing operations of the kind set out above, and thereby facilitating the work of the doctor and the medical staff and achieving a higher degree of safety for the patient.
In the past, it has already been usual to convey fluids under sterile conditions by using hose or tube pumps, preferably of the so-called roller pump type, as in this kind of pump the conveyor medium only comes into contact with the pump tube, which is particularly easy and cheap to produce as a sterile article intended to be used only once. However, these pumps are not readily suitable for precise control of quantities of fluid. Important factors for this purpose are inter alia the tolerances in respect of the internal diameter of the tube and thus the internal cross-sectional area of the tube. Even when the tubes are produced particularly carefully, according to manufacturers' information, the diameter tolerances are of the order of from 2 to 3% with the usual extrusion method, so that the cross-sectional area and thus the amount delivered per unit of time already suffers from a deviation of the order of from 4 to 6%. Further deviations can result from the fact that the tube cannot be fitted into the pump head in a manner which can be precisely reproduced, for example because the tube is twisted or installed with a different pre-stressing. A further reason for an inaccurate metering operation is the fatigue from which the tube material suffers in the passage of time and which results in the tube no longer returning completely to its original shape after it has been relieved of load. This therefore results in a reduction in the internal cross-sectional area and a fall in pumping output. Finally, the pressure obtaining at the input, on the suction side of the pump, should be mentioned as a particularly serious influence. Irrespective of the resilient properties of the tube, there will be a different degree of filling of the tube, which will thus influence the pump output, when the pressure at the pump input varies. This effect can assume considerable proportions particularly when the input pressure of the pump is below atmospheric pressure.
The factors listed above make it seem at first sight as though there is not a great deal of prospect for success in using tube pumps for the desired fluid balancing action, in view of the requirement that the balancing error should if possible not be more than 0.5% for example in a haemofiltration process. However, while using tube pumps, the invention provides steps whereby the fluid balancing error is readily reduced to the required degree and is even reduced to a substantially greater degree.